Wireless communication system, wireless communication apparatus, wireless communication method, and computer program

ABSTRACT

Random access operation is performed under a communication environment in which a plurality of communication modes having different transmission rate coexist with small overhead. A high-grade communication station spoofs information of a packet length and a rate in a decoding portion so that a value of (packet length)/(rate) corresponds to a duration where the communication is hoped to be stopped. The other station receiving the spoofed information receives the rest of the packet with the designated rate during the interval designated by the value of (packet length)/(rate). In this case, the packet length and the rate are not those of actually transmitted packet so that this packet is discarded.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of and is based upon and claims thebenefit of priority under 35 U.S.C. §120 for U.S. Ser. No. 12/426,478,filed Apr. 20, 2009 which is a continuation of U.S. Ser. No. 10/910,646,filed Aug. 4, 2004, and claims the benefit of priority under 35 U.S.C.§119 from Japanese Patent Application No. 2004-003530, filed Jan. 8,2004, the entire contents of each which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless communication system, awireless communication apparatus, a wireless communication method, and acomputer program for performing mutual communication among a pluralityof wireless stations like a wireless local area network (LAN). Inparticular, the present invention relates to a wireless communicationsystem, a wireless communication apparatus, a wireless communicationmethod and a computer program in which each communication stationperforms random access on the basis of carrier detection in accordancewith the carrier sense multiple access with collision avoidance (CSMA)system.

To be more precise, the present invention relates to a wirelesscommunication system, a wireless communication apparatus, a wirelesscommunication method and a computer program for realizing random accessin a communication environment in which a plurality of communicationmodes each having a transmission rate different from each other isintermixed. In particular, the present invention relates to a wirelesscommunication system, a wireless communication apparatus, a wirelesscommunication method and a computer program for realizing random accesswith a smaller overhead under a communication environment in which aplurality of communication modes each having a transmission ratedifferent from each other is intermixed.

2. Description of the Related Art

By setting up a LAN by connecting a plurality of computers to eachother, the sharing of information such as a file and data, and thesharing of peripheral equipment such as a printer can be achieved, andfurther the exchange of information such as the transfer of electronicmail, data, contents and the like can be preformed.

Conventionally, a wired LAN connection using an optical fiber, a coaxialcable or a twisted-pair cable has been generally used. In this case,line construction work is needed, and it is difficult to set up anetwork easily. Furthermore, the laying of a cable is troublesome. Inaddition, after setting up a LAN, because the moving range of anapparatus is limited by the length of a cable, the wired LAN isinconvenient.

Accordingly, a wireless LAN is noticed as a system for releasing a userfrom LAN wiring of the wired system. Because almost all of wiring cablescan be omitted in a work space such as an office in case of the wirelessLAN, communication terminals such as personal computers (PC's) can berelatively easily moved.

In recent years, as the wireless LAN system has become high in speed andlow in cost, the demand of the wireless LAN has been remarkablyincreased. In particularly, in the most recent days, for performinginformation communication among a plurality of electronic apparatusexisting around a person by setting up a small-scale wireless networkamong them, the introduction of a personal area network (PAN) has beenexamined. For example, different wireless communication systems usingfrequency bands such as a 2.4 GHz band and a 5 GHz band which are notrequired to be licensed by the competent authorities to use have beendefined.

As normal standards with regard to the wireless network, Institute ofElectrical and Electronics Engineers (IEEE) 802.11 (see, for example,Non-Patent Document 1), High Performance Wireless Local Area Network(HIPERLAN)/2 (see, for example, Non-Patent Document 2 or Non-PatentDocument 3), IEEE 802.15.3, Bluetooth communication and the like can becited. The IEEE 802.11 standard includes various wireless communicationsystems such as an IEEE 802.11a standard and an IEEE 802.11b standardaccording to the differences of a wireless communication system, afrequency band to be used, and the like.

A method of providing an apparatus to be a control station called as an“access point” or a “coordinator” in an area to form a network under thegeneralized control by the control station for constituting a local areanetwork by means of a wireless technique is generally used.

A wireless network locating an access point therein widely adopts anaccess control method based on a band reservation, in which when acertain communication apparatus performs an information transmission,the communication apparatus first reserves a band necessary for theinformation transmission at an access point for using a transmissionpath in order not to generate any collisions with the informationtransmission of another communication apparatus. That is, the wirelessnetwork performs a synchronized wireless communication in which eachcommunication apparatus in the wireless network is synchronized witheach other by locating the access point.

However, there is a problem in which the usability of a transmissionpath is reduced to half when an asynchronous communication is performedbetween communication apparatus on a transmission side and a receptionside in a wireless communication system locating an access point thereinbecause the wireless communication through the access point is certainlynecessary.

On the other hand, as an another method for constituting a wirelessnetwork, an “ad-hoc communication” in which terminals are directlyperform wireless communications with each other asynchronously has beendevised. In particular, in a small-scale wireless network composed of arelatively few clients positioned near to each other, the ad-hoccommunication, by which arbitrary terminals can directly performasynchronous wireless communications with each other without using aspecific access point, is considered to be suitable.

Because there is no central control station in an ad-hoc type wirelesscommunication system, the system is suitable for constituting, forexample, a home network composed of household electric apparatus. Anad-hoc network has the following features. That is, even if a terminalis in trouble or the power source thereof is off, a routing can beautomatically changed, and consequently the network is difficult tobreak. Also, data can be transmitted relatively long distance whilekeeping a high-speed data rate by making a packet hop a plurality oftimes between mobile stations. Many development examples with regard tothe ad-hoc system are known (see, for example, Non-Patent Document 4).

For example, in an IEEE 802.11 series wireless LAN system, an ad-hocmode in which terminals operate in an autonomous distributed way in peerto peer without locating any control station is prepared.

Hereupon, it is necessary to avoid contention when a plurality of usersaccesses the same channel. As a typical communication procedure foravoiding the contention, carrier sense multiple access with collisionavoidance (CSMA) is known. The CSMA indicates a connection method ofperforming multiple access on the basis of carrier detection. Because itis difficult to receive a signal which a terminal itself has performedan information transmission thereof in a wireless communication, aterminal starts own information transmission after confirming thenonexistence of information transmissions of the other communicationapparatus not by a CSMA/collision detection (CD) method but by aCSMA/collision avoidance (CA) method for avoiding any collisions.

A communication method based on the CSMA/CA is described with referenceto FIG. 11. In the example shown in the drawing, it is supposed thatthere are four communication stations #0 to #3 under a certaincommunication environment.

Each communication station having transmission data monitors a mediumstate for a predetermined inter frame space, or a distributedcoordination function (DCF) inter frame space (DIFS), from the lastdetection of a packet. When any media are clear, namely when there areno transmission signals, the communication station performs randombackoff. Furthermore, when there are no transmission signals also inthis period, a transmission right is given to the communication station.

In the shown example, after monitoring the medium state for an interframe space DIFS, the communication station #0, which has the randombackoff set to be shorter than that of the other peripheral stations,acquires the transmission right to be able to start a data transmissionto the communication station #1.

At the data transmission, the communication station #0, or thetransmission source, stores the information for a network allocationvector (NAV), and describes a period of time until the completion of thetransaction of a data communication in a duration field of the header ofa MAC frame (MAC header).

The communication station #1, or the transmission destination of thedata frame, performs a reception operation of the data addressed to thelocal station for the duration of the Duration described in the MACheader. When the data reception has been completed, the communicationstation #1 returns an ACK packet to the communication station #0, or thedata transmission source.

Moreover, the communication stations #2 and #3, which have received thedata frame, and which are not the data transmission destinations, decodethe description in the Duration field of the MAC header, and recognizethe state in which the medium is occupied without monitoring the mediumuntil the transaction ends to stop the transmission. The work is calledthat the peripheral stations “raise a NAV”, or the like. The NAV iseffective over the duration indicated in the Duration field. Forexample, the duration until the communication station #1, or thereception destination, will return the ACK packet is specified as theDuration.

In such a way, according to the CSMA/CA system, contention is avoidedwhile a single communication station acquires a transmission right, andwhile peripheral stations stop their data transmission operations duringthe duration of the data communication operation, and thereby collisionscan be avoided.

Hereupon, it is known that a concealed terminal problem is generated ina wireless LAN network in an ad hoc environment. The concealed terminalindicates a communication station which a communication station on oneside of a communication party can hear but a communication station onthe other side of the communication party cannot hear in case ofperforming a communication between certain specific communicationstations. Because no negotiations can be performed between concealedterminals, there is the possibility that transmission operations collidewith each other only by the above-mentioned CSMA/CA system.

A CSMA/CA in accordance with an RTS/CTS procedure is known as amethodology for solving the concealed terminal problem. Also in the IEEE802.11, the methodology is adopted.

In an RTS/CTS system, a data transmission source communication stationtransmits a transmission request packet Request To Send (RTS), andstarts a data transmission in response to the reception of aconfirmation note packet Clear To Send (CTS) from a data transmissiondestination communication station. Then, when a concealed terminalreceives at least one of the RTS and the CTS, the concealed terminalsets a transmission stop duration of the local station for the durationin which the data transmission based on the RTS/CTS procedure isexpected to be performed, and thereby collisions can be avoided. Theconcealed terminal for a transmission station receives the CTS to set atransmission stop duration for avoiding the collision with a datapacket. The concealed terminal for a reception station receives the RTSto stop the transmission duration for avoiding the collision with theACK.

FIG. 12 shows an operation example of the RTS/CTS procedure.Incidentally, it is supposed that there are four communication stations#0 to #3 in the communication environment of the wireless communicationenvironment. The communication stations #0 to #3 are supposed to be inthe following state. That is, the communication station #2 cancommunicate with the adjacent communication station #0. Thecommunication station #0 can communicate with the adjacent communicationstations #1 and #2. The communication station #1 can communicate withthe adjacent communication stations #0 and #3. The communication station#3 can communicate with the adjacent communication station #1. However,the communication station #2 is a concealed terminal for thecommunication station #1, and the communication station #3 is aconcealed terminal for the communication station #0.

Each communication station having transmission data monitors a mediumstate for a predetermined inter frame space DIFS (DCF Inter Frame Space)until the communication station has detected a packet last. When themedium is clear, namely when the there are no transmission signals,during this period of time, the communication station performs randombackoff. Moreover, when there are no transmission signals also duringthis period of time, the communication station is given a transmissionright.

In the example shown in the drawing, the communication station #0, whichhas set the random backoff shorter than that of the other peripheralstations after the monitoring of the medium state for the inter framespace DIFS, can acquire the transmission right to start the datatransmission to the communication station #1.

That is, the communication station #0, which transmits data, transmits atransmission request packet (RTS) to the communication station #1. Onthe other hand, the communication station #1 being the receptiondestination returns a confirmation note (CTS) to the communicationstation #0 after a shorter inter frame space Short IFS (SIFS). Then, thecommunication station #0 responds to the reception of the CTS packet tostart the transmission of a data packet after the inter frame spaceSIFS. Moreover, when the communication station #1 completes thereception of the data packet, the communication station #1 returns anACK packet with an inter frame space SIFS put between. Because the interframe space SIFS is shorter than the inter frame space DIFS, thecommunication station #1 can transmit the CTS packet before the otherstations, which acquires the transmission right after waiting forDIFS+random backoff in accordance with a CMSA/CA procedure.

At this time, the communication station #2 and the communication station#3, both located at positions where both of them can be concealedterminals from both of the communication station #0 and thecommunication station #1, performs control to detect the use of atransmission path by the reception of the RTS or the CTS, and not toperform any transmissions until the communication ends.

To put it more specific, the communication station #2 detects the startof the data transmission of the communication station #1 as thetransmission source on the basis of an RTS packet, and decodes theDuration field described in the MAC header of the RTS packet, andfurther recognizes that the transmission path has been already usedafter that for the duration until the successive transmission of thedata packet is completed (the duration until the end of ACK). Thereby,the communication station #2 can raise a NAV.

Moreover, the communication station #3 detects the 25 start of the datatransmission of the communication station #1 as the receptiondestination on the basis of the CTS packet, and decodes the Durationfield described in the MAC header of the CTS packet, and furtherrecognizes that the transmission path has been already used after thatduring the duration until the transmission of the successive data packetis completed (the duration until the ACK had ended). Thereby, thecommunication station #3 can raise a NAV.

In such a way, when a concealed terminal receives at least one of theRTS and the CTS, the concealed terminal sets the transmission stopduration of the local station for the duration to be expected to performthe data transmission based on the RTS/CTS procedure. Consequently,collisions can be avoided.

Now, the standardization of the IEEE 802.11g for supporting higher speedcommunication rate as a higher rank standard of the IEEE 802.11b being awireless LAN specification using 2.4 GHz band has been advanced. Acommunication station in accordance with the IEEE 802.11g (hereinafteralso referred to “high-grade communication station” simply) can alsooperate in accordance with the IEEE 802.11b, and can transmit a datapacket also at a high-speed rate at which a conventional communicationstation in accordance with the IEEE 802.11b (hereinafter also referredto as “conventional station” simply) cannot perform any reception.

Hereupon, there is a problem of the coexistence of differentcommunication systems, or a problem of the coexistence of the IEEE802.11g and the IEEE 802.11b, both using the same band. That is, becausethe conventional station cannot receive a data packet to be transmittedat a high-speed rate, the conventional station cannot decode theDuration described in the MAC header, and cannot raise a NAVappropriately. Consequently, the conventional station cannot avoidcollisions.

For example, in the example shown in FIG. 11, the communication station#0 and the communication station #1, both being communication parties,can exchange a data packet at a high-speed rate in conformity with IEEE802.11g. On the other hand, when the communication station #2 and thecommunication station #3 around the communication station #0 and thecommunication station #1 are conventional stations which do not conformto the IEEE 802.11g, the communication stations #2 and #3 cannot decodethe Duration described in the MAC header as a result of being unable toreceive the data packet. Consequently, there is the possibility that thecommunication stations #2 and #3 start their communication operationeven in the duration of the Duration to generate a collision (see FIG.13).

The present inventors consider that the problem of the coexistence ofthe IEEE 802.11g and the IEEE 802.11b is preferably solved by thesetting of the IEEE 802.11g, being a higher rank standard, to assuread-hoc compatibility.

For example, a method of performing the exchange of an RTS/CTS packet ata transmission rate at which a conventional station can receive theRTS/CTS packet before the transmission of a data packet in IEEE 802.11gcan be considered (see FIG. 14). In this case, peripheral conventionalstations decodes the Duration field described in the MAC header of theRTS/CTS packet, and recognize that the transmission path has alreadyused for the duration until the completion of the transmission of thesuccessive data packet after that (the duration until ACK ends).Thereby, the peripheral conventional stations can raise an NAV only forsuitable duration. That is, the conventional stations cannot hear a datapacket to be transmitted at a high-speed rate, but that turns to be noproblem for avoiding a collision.

A procedure for securing a band in accordance with the above-mentionedprocedure before the transmission of a data packet is generally called avirtual carrier sense.

However, in such a band securing procedure, the transmission of a datapacket cannot be performed without performing the RTS/CTS procedurecertainly not only in the case where the concealed terminal problem isgenerated, but also in the case where the concealed terminal problemdoes not exist. That is, the faster the transmission rate becomes, thelarger the problem of an RTS/CTS overhead becomes. Also, thecommunication efficiency decreases by the degree of the problem.

Non-Patent Document 1: International Standard ISO/IEC 8802-11: 1999(E)ANSI/IEEE Std. 802.11, 1999 Edition, Part 11: Wireless LAN Medium AccessControl (MAC) and PHYsical Layer (PHY) Specifications.

Non-Patent Document 2: ETSI Standard ETSI TS 101 761-1 V1. 3.1 BroadbandWireless Access Networks (BRAN); HIPERLAN Type 2; Data Link Control(DLC) Layer; Part 1: Basic Data Transport Functions.

Non-Patent Document 3: ETSI TS 101 761-2 V1. 3.1 Broadband WirelessAccess Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer;Part 2: Wireless Link Control (RLC) sublayer.

Non-Patent Document 4: C. K. Tho, “Ad-Hoc Mobile Wireless Network”(Prentice Hall PTR Corp.).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a superior wirelesscommunication system, a wireless communication apparatus, a wirelesscommunication method, and a computer program in which each communicationstation can suitably perform random access by the CSMA system on thebasis of carrier detection.

It is another object of the present invention to provide a superiorwireless communication system, a wireless communication apparatus, awireless communication method, and a computer program which can realizerandom access in a communication environment in which a plurality ofcommunication modes each having a different transmission rate to eachother intermixes.

It is a further object of the present invention to provide a superiorwireless communication system, a wireless communication apparatus, awireless communication method, and a computer program which can realizerandom access with a smaller overhead in a communication environment inwhich a plurality of communication modes each having a differenttransmission rates to each other intermixes.

The present invention was made in consideration of the above-mentionedproblems. A first aspect of the present invention is a wirelesscommunication system in which a first communication station operatingaccording to a first communication method and a second communicationstation capable of operating according to both of the firstcommunication method and a second communication method coexist, whereinthe second communication station transmits a packet composed of a firstdecoding portion capable of being received according to the firstcommunication method, and a second decoding portion capable of beingreceived according to the second communication method.

In this case, the “system” hereupon indicates a matter made of aplurality of logically aggregate apparatus (or logically aggregatefunctional modules realizing specific functions), and it does not matterwhether each of the apparatus or the functional modules is in a singlehousing or not.

Moreover, the first communication method hereupon corresponds to, forexample, the IEEE 802.11b being a wireless LAN specification using a 2.4GHz band, and the second communication method corresponds to the IEEE802.11g supporting a high-speed communication rate as a higher rankstandard of the IEEE 802.11b.

Under such communication environment, there is a problem of thecoexistence of the IEEE 802.11g and the IEEE 802.11b, both using thesame frequency band.

For example, when a transmission and a reception of a packet isperformed by random access, for example, the local station transmits adata packet as a data transmission station, and hopes that peripheralstations stop their communication operations for expected duration untilan ACK is returned from a reception station. Moreover, when the RTS/CTSprocedure is adopted, for example, the local station transmits an RTS ora CTS packet, and hopes that the peripheral stations stop theircommunication operations for the expected duration until the ACK isreturned. However, when the second communication station operating inaccordance with the higher rank standard performs a packet transmissionaccording to the second communication method, a conventional stationcannot receive the data packet transmitted at a high-speed rate, andcannot decode a duration described in a MAC header. Then, theconventional station cannot raise a NAV suitably, and cannot avoid acollision.

In the wireless communication system according to the present invention,a packet is composed of a first decoding portion capable of beingreceived according to a first communication method, and a seconddecoding portion capable of being received according to a secondcommunication method. The first decoding portion includes informationrelated to a packet length and a transmission rate of the packet.Further, the first communication station that receives the packetcalculates (packet length)/(transmission rate) on the basis of thepacket length and the transmission rate of the packet, both capable ofbeing obtained by decoding the first decoding portion, in order toobtain a residual reception period of time of the packet.

Then, when the second communication station performs a communicationprocedure according to the second communication method, the secondcommunication station describes spoofed information of a packet lengthand a transmission rate in the first decoding portion like theindication of the duration for which communication operations of theother stations are stopped by (packet length)/(transmission rate) forthe sake of the communication procedure. In such a case, the firstcommunication station cannot receive the second decoding portion of thepacket, but can avoid a collision by calculating the (packetlength)/(transmission rate) on the basis of the description in the firstdecoding portion to raise the NAV for desired duration, and by stoppingany data transmissions.

That is, in the wireless communication system according to the presentinvention, the second communication station performing a packettransmission spoofs about the information of the packet length and thetransmission rate to be described in the first decoding portion in orderthat the first communication station receiving the packet may stop itscommunication operation for the duration until a communicationtransaction to be performed according to the second communication methodends. Thereby, the second communication station performing the secondcommunication method realizes the so-called upper compatibility to thefirst communication station.

The duration until the communication transaction ends hereupon,specifically indicates the duration until an ACK transmission ends in acommunication procedure preformed according to the second communicationmethod. Moreover, when a packet transmission is performed in accordancewith a communication procedure to perform multiple connections with aplurality of communication stations in the second decoding portion, theduration hereupon indicates the duration until all of the ACKtransmissions performed in a time division multiplex from each remotestation end. Moreover, the transmission of the ACK packet hereupon isnot limited to the case of single ACK packet, but includes, for example,the case where the ACK packet is multiplexed with other kinds of packetssuch as an RTS packet, a CTS packet, and data packet to be transmitted.

For the second communication station described above realizes themechanism of the ad-hoc compatibility, it is necessary for each secondcommunication station to recognize that the information of the packetlength and the transmission rate described in the first decoding portionis spoofed. Moreover, it is necessary that each second communicationstation mutually recognizes the spoofing of the information while thefirst communication station cannot know the spoofing of the informationto operate in accordance with the description in the first decodingportion.

Accordingly, in the wireless communication system according to thepresent invention, the second communication station performing a packettransmission describes whether the spoofed information of a packetlength and a transmission rate is described in the first decodingportion or not in a packet in a format which the second communicationstation capable of operating according to the second communicationmethod can decode the spoofed information but the first communicationstation operating according to the first communication method cannotdecode the spoofed information.

For example, the second communication station performing a packettransmission indicates whether the spoofed information of the packetlength and the transmission rate is described or not by means of aspoofed flag in the first decoding portion.

In this case, when the second communication station being a datareception side detects that the information of the packet length and thetransmission rate in the first decoding portion of a packet receivedfrom another station is spoofed, the second communication stationswitches its reception method to the second communication method, andcan perform the reception operation of the residual portion of thepacket.

Moreover, the second communication station performing a packettransmission may be provided with a known second communication methoddecoding portion, in which all of the second communication stations candecode data, in a packet, and may describe whether spoofed informationof a packet length and a transmission rate is described or not in thesecond communication method decoding portion for notifying the othersecond communication stations of the spoofing. For example, when aplurality of communication modes each having a transmission ratedifferent from each other is defined as the second communication method,an actually used communication mode may be described in the secondcommunication method decoding portion.

It is preferable that a second communication station performing a packettransmission transmits the second communication method decoding portionin a communication method in which all of the second communicationstations can decode the data in the second communication method decodingportion but the first communication stations cannot decode the data. Forexample, the second communication station performing the packettransmission transmits the second communication method decoding portionat a low transmission rate of about 6 Mbps in order that all of thesecond communication stations can receive, but the second communicationstation performing the packet transmission performs the modulationprocessing of the second communication method decoding portion inaccordance with a modulation system which each of the secondcommunication stations knows but the first communication stations do notknow. Thereby, only the second communication stations can demodulate thesecond communication method decoding portion to recognize that the firstdecoding portion is spoofed.

In such a case, a second communication station receiving the packettries to decode the second communication method decoding portion bymeans of both of the first communication method and the communicationmethod by which the first communication station cannot decode the secondcommunication method decoding portion, and the second communicationstation can recognize that the first decoding portion is spoofed by thefact that the second communication method decoding portion can bedecoded according to the latter method. Then, the second communicationstation can performs the reception processing of the second decodingportion in accordance with the communication mode obtained from thesecond communication method decoding portion.

For example, the second communication station locates the secondcommunication method decoding portion before the second decoding portionin a packet. Then, when the second communication station describes thespoofed information of a packet length and a transmission rate for thefirst communication stations in the first decoding portion, the secondcommunication station describes the information related to an actualpacket length and a transmission rate in the second decoding portion inthe second communication method decoding portion. In such a case, asecond communication station receiving the packet can perform thereception operation of the second decoding portion after the secondcommunication method decoding portion of the received packet on thebasis of the information related to the packet length and thetransmission rate described in the second communication method decodingportion.

A second communication station performing a packet transmission can makedata to be able to be decoded by all of the second communicationstations and to be unable to be decoded by the first communicationstations by modulating the second communication method decoding portionin accordance with a modulation system which only each of the secondcommunication station knows. For example, when the second communicationstation performs a phase modulation such as BPSK to the secondcommunication method decoding portion, the second communication stationmay give a phase difference θ, which is jointly owned by the secondcommunication station, to the location of a signal point (−1, 1), or maytranslation the signal point by the known quantity Δd. On the otherhand, a second communication station receiving the packet performs phasedemodulation in consideration of the phase shifts of the location of thesignal point such as the phase difference −θ, the movement quantity −Δdand the like. Then, it can be known that the first decoding portion isspoofed by the fact that the second communication method decodingportion can be decoded.

Incidentally, in the case where a second communication station capableof operating according to the second communication method is located ata position far from a transmission source, a situation in which thesecond communication station can receive a second communication methoddecoding portion, which is transmitted at a low transmission rate, butcannot receive the second decoding portion, which is transmitted at ahigh-speed transmission rate, owing to an S/N, can be also supposed. Insuch a case, a second communication station receiving a packet tries toperform the reception operation of a second decoding portion on thebasis of the information related to a packet length and a transmissionrate described in the second communication method decoding portion ofthe received packet. When the second communication station cannot decodethe second decoding portion, the second communication station may obtaina difference between a period of time (i.e. (packetlength)/(transmission rate)) obtained from the spoofed packet length andthe transmission rate described in the first decoding portion and aperiod of time (i.e. (packet length)/(transmission rate)) obtained fromthe packet and the transmission rate described in the secondcommunication method decoding portion, and may restrain the transmissionof a packet for a predetermined period of time.

The wireless communication system according to the present inventionsupposes, for example, a communication environment in which aconventional station operating in accordance with the IEEE 802.11b and ahigh-grade communication station operating in conformity with the IEEE802.11g corresponding to a high-speed edition standard using the sameband intermixedly operate.

In the wireless communication system according to the present invention,a packet to be transmitted is composed of a known fixed rate portion(hereinafter also referred to as “general decoding portion”) which allof the communication stations can decode, and an arbitrary rate portion(hereinafter also referred to as “high-grade decoding portion”) whichpossibly only a part of the communication station being at a high-gradecan decode.

The general decoding portion of a packet generally describes a residuallength of the packet and a rate at which residual packets aretransmitted therein. Consequently, a communication station receiving thepacket tries to receive the residual part of the packet by performingthe reception operation of the packet at a specified rate for theduration of (packet length)/(rate).

In the present invention, a high-grade communication station performs apacket transmission at a transmission rate at which a conventionalstation cannot receive the packet. Also, when a conventional station isnot desired to start a transmission for fixed duration, the informationof a packet length and a rate in the general decoding portion is spoofedin order that the value of (packet length)/(rate) may be the durationfor which the communication is desired to be stopped. For example, thevalue of (packet length)/(rate) should originally correspond to thereception duration of the residual portion of the packet. However, forexample, the information is spoofed in order to be the duration forwhich a NAV such as the end of ACK should be raised.

Moreover, in this case, the high-grade communication station to be acommunication party is needed to detect that these values described inthe general decoding portion are spoofed for performing a correctreception operation without performing any malfunctions on the basis ofthe spoofed rate and the length. For this sake, a flag indicating theexistence of spoofing is provided in the general decoding portion of apacket. Alternatively, a second communication method decoding portion,which all second communication stations can decode, is provided, and thefact that the general decoding portion is spoofed is described in thesecond communication method decoding portion. Then, after the generaldecoding portion has been transmitted, the high-grade communicationstation shifts to an arbitrary high grade rate mode, and transmits anactual data composed of a high-grade decoding portion.

When the conventional station receives a general decoding portionincluding the spoofed information of a packet length and a rate, theconventional station believes the packet length and the rate to receivethe residual packet at a specified rate for a period of (packetlength)/(rate). Because the rate and the packet length are differentfrom ones at which the packet is actually transmitted, the conventionalstation cannot decode the packet correctly, and the packet is destroyed.

On the other hand, a high-grade communication station detects that theinformation of a packet length and a rate is spoofed by means of theflag in the general decoding portion. Alternatively, the high-gradecommunication station detects the spoofing owing to the capability ofdecoding the second communication method decoding portion. Then, whenthe general decoding portion is spoofed, the high-grade communicationstation shifts to the corresponding high grade rate mode, and receivesthe residual packet, i.e. a high-grade decoding portion. Thus, thehigh-grade communication station can decode actual data.

As described above, in the case where a packet length and a rate areused for setting a period of time during which all transmission startsare stopped, there are plurality of combinations of spoofed packetlengths and spoofed rates for showing the same period of time to theconventional station. On the other hand, there is a plurality oftransmission modes as a high-speed communication rate sometimes.Accordingly, when a plurality of modes each including high-speedcommunication rate, a mode by which the residual packet is transmittedmay be presumed by the setting of a rate.

Moreover, a second aspect of the present invention is a computer programdescribed in a form capable of being read by a computer to execute on acomputer system processing for a wireless communication operation in awireless communication environment in which a first communication methodand a second communication method coexist, the program including thesteps of: generating a transmission packet composed of a first decodingportion and a second decoding portion, transmitting a first decodingportion of the transmission packet according to the first communicationmethod, and transmitting a second decoding portion of the transmissionpacket according to the second communication method, receiving andanalyzing a first decoding portion of a reception packet from anotherstation, and receiving and analyzing a second decoding portion of thereception packet according to the second communication method.

The computer program according to the second aspect of the presentinvention defines a computer program described in the form capable ofbeing read by a computer for realizing predetermined processing on acomputer system. In other words, by installing the computer programaccording to the second aspect of the present invention into a computersystem, a cooperative operation is exhibited on the computer system, andthe computer system operates as a wireless communication apparatus. Bybuilding a wireless network by activating a plurality of such wirelesscommunication apparatus, operations and advantages similar to those ofthe wireless communication system according to the first aspect of thepresent invention can be obtained.

According to the present invention, it is possible to provide a superiorwireless communication system, a wireless communication apparatus, awireless communication method and a computer program in which eachcommunication station can suitably perform random access on the basis ofa carrier detection according to a CSMA system.

Moreover, according to the present invention, it is possible to providea superior wireless communication system, a wireless communicationapparatus, a wireless communication method and a computer program inwhich random access can be realized in a communication environment inwhich a plurality of communication modes each having a transmission ratedifferent from each other intermixes.

Moreover, according to the present invention, it is possible to providea superior wireless communication system, a wireless communicationapparatus, a wireless communication method and a computer program inwhich random access can be realized with a smaller overhead in acommunication environment in which a plurality of communication mode,each having a transmission rate different from each other, intermixes.

According to the present invention, the coexistence of the IEEE 802.11gand the IEEE 802.11a/b, both using the same band, can be realizedwithout passing an RTS/CTS procedure. Consequently, an overhead can beremarkably reduced.

Moreover, according to the present invention, the duration for an NAVcan be flexibly set. Consequently, the throughput of a system can beimproved.

Further objects, features and advantages of the present invention willbe apparent by more detailed descriptions based on the embodiments ofthe present invention and the attached drawings, which will be describedlater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing a functional configuration of awireless communication apparatus operating as a communication station ina wireless network according to one embodiment of the present invention;

FIG. 2 is a view for illustrating a mechanism of a priority transmissionbased on difference of inter frame spaces;

FIG. 3 is a view schematically showing one example of a frameconfiguration of a packet in the wireless network according to thepresent invention;

FIG. 4 is view schematically showing a variation of a packet structurein the wireless network according to the present invention;

FIG. 5 is a view showing a description example of Rate field in the IEEE802.11a;

FIG. 6 is a flowchart showing a reception processing procedure in thecase where a wireless communication apparatus 100 operates as aconventional station;

FIG. 7 is a flowchart showing a reception processing procedure in thecase where the wireless communication apparatus 100 operates as ahigh-grade communication station;

FIG. 8 is a view showing one of communication operation examples basedon CSMA/CA according to the present invention;

FIG. 9 is a view showing one of communication operation examples basedon RTS/CTS according to the present invention;

FIG. 10 is a view showing one of communication operation examples basedon RTS/CTS using Shot NAV according to the present invention;

FIG. 11 is a view showing a communication operation example base onCSMA/CA according to a conventional technology;

FIG. 12 is a view showing a communication operation example based onRTS/CTS according to a conventional technology;

FIG. 13 is a view showing a communication operation example based onCSMA/CA under a communication environment in which conventional stationsand high-grade communication stations intermix according to aconventional technology;

FIG. 14 is a view showing a communication operation example based onRTS/CTS in conformity with the IEEE 802.11g according to a conventionaltechnology;

FIG. 15 is a view showing one example of the internal configuration of awireless reception unit 110 of a high-grade communication stationcapable of decoding a SIGNAL-2 portion;

FIG. 16 is a view showing one example of the frame configuration of apacket transmitted according to the second communication method; and

FIG. 17 is a view showing communication operation sequencing by which aplurality of reception stations replies by a time division responsepacket to a transmission packet from a transmission station.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention are described in detailhereinafter with reference to the attached drawings.

Channels of communication supposed in the present invention are wirelesschannels, and a network is built among a plurality of communicationstations. Communication supposed in the present invention is a store andforward type traffic, and information is transferred per packet.Moreover, although each communication station is supposed to have asingle channel in the following description, it is also possible toexpand the description to the case where a transmission medium composedof a plurality of frequency channels, i.e. multi channels, is used.

In a wireless network according to the present invention, eachcommunication station directly (randomly) transmits information inaccordance with an access procedure based on a carrier sense multipleaccess (CSMA) (carrier detection multiple connection), and can build anautonomous distributed type wireless network. Moreover, in the wirelessnetwork according to the present invention, transmission control usingchannel resources effectively is performed by means of transmission(MAC) frame in a gentle time division multiplexing access structure. Inthis case, each communication station can perform an access system basedon a time synchronization such as reserving a frequency band and settinga priority use duration.

One embodiment of the present invention supposes, for example, acommunication environment in which both high-grade communicationstations in conformity with the IEEE 802.11g corresponding to ahigh-speed edition standard using the same band and a conventionalstation in conformity with the conventional IEEE 802.11b intermixedlyoperate. That is, there are two kinds of communication terminals, thatis; conventional stations which can transmit and receive only thepackets modulated according to some limited modulation systems; and highgrade communication stations which can receive packets according to ahigh-grade system in addition to the modulation system by which theconventional station can receive packets.

The communication system in which the IEEE 802.11g and the IEEE 802.11b,both using the same band, intermix has a problem of coexistence. Thereason is that, because the conventional station cannot receive a datapacket transmitted at a high-speed rate, the conventional station cannotdecode the Duration described in a MAC header to raise an NAV suitably,and consequently cannot avoid a collision. The present invention solvesthe coexistence problem by securing that the higher rank standard IEEE802.11g assures the conventional standard IEEE 802.11b of the so-calledupper compatibility. The solving method will be described later.

FIG. 1 schematically shows a functional configuration of a wirelesscommunication apparatus operating as a communication station in awireless network according to one embodiment of the present invention. Awireless communication apparatus 100 shown here can form a network whileavoiding a collision in the same wireless system by performing a channelaccess effectively. The wireless communication apparatus 100 is eitherof a conventional station in conformity with the IEEE 802.11a/b as afirst communication method and a high-grade communication station inconformity with the IEEE 802.11g as a second communication method.

As shown in FIG. 1, the wireless communication apparatus 100 is composedof an interface 101, a data buffer 102, a central control unit 103, apacket generation unit 104, a wireless transmission unit 106, a timingcontrol unit 107, an antenna 109, a wireless reception unit 110, apacket analysis unit 112, and an information storage unit 113.

The interface 101 performs exchanges of various kinds of informationbetween the wireless communication apparatus 100 and an externalapparatus (such as a personal computer, though not shown) connected tothe wireless communication apparatus 100.

The data buffer 102 is used for temporarily storing the data transmittedfrom the external apparatus connected to the wireless communicationapparatus 100 through the interface 101, and the data received through awireless transmission path before transmitting the data through theinterface 101.

The central control unit 103 unitarily performs the administration ofseries of information transmission and reception processing in thewireless communication apparatus 100 and the access control oftransmission paths. Basically, the central control unit 103 sets a timerof backoff to operate over a random period of time on the basis of CSMAwhile monitoring the states of the transmission paths, and performsaccess contention of acquiring a transmission right in the case where notransmission signals exist during this period of time.

The present embodiment adopts a mechanism of a priority transmission inthe access contention to realize flexible QoS (see FIG. 2). For example,the wireless communication apparatus 100 takes a normal operation modeafter a packet transmission of another station or at the time of lowtraffic priority, and sets an inter frame space IFS to a longer DIFS,and further sets the random backoff. On the other hand, in case ofperforming the transmission of CTS successively to RTS from anotherstation, in case of performing the transmission of a data packetsuccessively to CTS, and in case of the transmission of ACK, thewireless communication apparatus 100 sets the inter frame space IFS to ashorter SIFS, which enables a transmission prior to the other stationsperforming normal transmission operations.

The packet generation unit 104 generates a packet signal to betransmitted from the local station to a peripheral station. The packethereupon includes a transmission request packet RTS from a communicationstation being a reception destination, a confirmation response packetCTS to the transmission request packet RTS, an ACK packet and the likeas well as a data packet. For example, a data packet is generated bytaking out of the transmission data stored in the data buffer 102 for apredetermined length to be set as a payload.

In a MAC layer of a communication protocol, a MAC frame is configured byadding a MAC header to a payload, and furthermore a PHY header is addedat a PHY layer to be a final transmission packet structure. In thepresent embodiment, the PHY header constitutes a first decoding portion,and the MAC frame portion constitutes a second decoding portion. Theconfiguration of a packet signal will be described later.

The wireless transmission unit 106 and the wireless reception unit 110correspond to an RF layer and the PHY layer in the communicationprotocol.

The wireless transmission unit 106 performs the wireless transmissionprocessing of a packet signal according to a predetermined modulationsystem and a transmission rate. To put it more specific, the wirelesstransmission unit 106 includes a modulator for modulating a transmissionsignal according to the predetermined modulation system, a D/A converterfor converting a digital transmission signal into an analog signal, anup-converter for performing the frequency conversion of an analogtransmission signal to up-convert the analog transmission signal, apower amplifier (PA) for amplifying the electric power of theup-converted transmission signal (all of them are not shown). Thewireless transmission unit 106 performs the wireless transmissionprocessing at a predetermined transmission rate.

Moreover, the wireless reception unit 110 performs the wirelessreception processing of the packet signal from another station. To putit more specific, the wireless reception unit 110 is composed of a lownoise amplifier (LNA) for amplifying the voltage of a wireless signalreceiving from another station through the antenna 109, a down-converterfor down-converting the voltage-amplified reception signal by frequencyconversion, an automatic gain controller (AGC), an A/D converter forperforming the digital conversion of an analog reception signal, ademodulator for performing a synchronous processing for acquiring asynchronization, a channel estimation, a demodulation processing bymeans of a demodulation system such as OFDM, and the like (all of themare not shown).

In the case where the wireless communication apparatus 100 conforms tothe IEEE 802.11a/b as the first communication method, the wirelesstransmission unit 106 and the wireless reception unit 110 respectivelyperform a transmission and a reception of a packet according to amodulation system and a transmission rate in conformity with a wirelessLAN standard. Moreover, in the case where the wireless communicationapparatus 100 conforms to the IEEE 802.11g as the second communicationmethod, it is possible for the wireless communication apparatus 100 toperform a transmission and reception of a packet according to amodulation system and a transmission rate in conformity with the IEEE802.11a/b. In addition, the wireless communication apparatus 100 canperform a transmission and a reception of a packet at a transmissionrate inherent to the IEEE 802.11g (i.e. a transmission and a receptionunable to be received by the IEEE 802.11a/b). In the latter case, thefirst decoding portion of a packet composed of the PHY header istransmitted and received at a transmission rate capable of beingreceived by the IEEE 802.11a/b, but the second decoding portion composedof the MAC frame is transmitted and received at a transmission rate inconformity with the IEEE 802.11g.

The antenna 109 performs the wireless transmission of a signal toanother wireless communication apparatus on a predetermined frequencychannel, or collects a signal transmitted from another wirelesscommunication apparatus. The present embodiment is provided with asingle antenna, and it is supposed that a transmission and a receptioncannot simultaneously performed in parallel.

The timing control unit 107 controls a timing for transmitting andreceiving a wireless signal. For example, the timing control unit 107controls its own packet transmission timing, the transmission timing ofeach packet (such as RTS, CTS, data, and ACK) in conformity with theRTS/CTS system (setting of an inter frame space IFS and the backoff),the setting of the NAV at the time of reception of a packet addressed toanother station, and the like.

The packet analysis unit 112 analyzes the packet signal which can bereceived from another station. In the present embodiment, the packet iscomposed of a first decoding portion and a second decoding portion. Thedetails of a packet decoding method will be described later.

The information storage unit 113 stores an execution procedureinstruction of a series of access control operations to be executed bythe central control unit 103, and information obtained from an analysisresult of a reception packet.

As described above, in a wireless network of the present embodiment,there are two kinds of communication stations of conventional stationscapable of the transmission and the reception of a packet modulatedaccording to some limited modulation systems, and high gradecommunication stations capable of the reception in conformity with ahigh-grade system in addition to the modulation systems in which theconventional stations can perform receptions. There is a coexistenceproblem in a communication system in which the IEEE 802.11g and the IEEE802.11b using the same band intermix. The present embodiment solves thisproblem by making the high-grade communication stations provide theso-called ad-hoc compatibility to the conventional stations. The detailsof the solution will be described.

FIG. 3 schematically shows the configuration of a packet which thewireless communication apparatus 100 operating as a communicationstation in the wireless network of the present embodiment transmits andreceives.

In a MAC layer of the communication protocol, a MAC frame is constitutedby adding a MAC header to a payload (corresponding to an IP packet).Moreover, in a PHY layer, a PHY header is added to the MAC frame to be afinal transmission packet structure. The PHY header constitutes a firstdecoding portion, and the MAC frame portion constitutes a seconddecoding portion. As shown in FIG. 3, a packet is composed of a physicallayer convergence protocol (PLCP) preamble portion and a SIGNAL portionas the PHY header, and the MAC frame. The MAC frame is composed of theMAC header and a data portion.

The PHY header corresponds to the first decoding 30 portion, and the MACframe corresponds to the second decoding portion.

In the case where the transmission station of a packet is a conventionalstation in conformity with the IEEE 802.11a/b, both of the PHY headerand the MAC frame are transmitted according to the first communicationmethod.

Moreover, in the case where the transmission station of a packet is ahigh-grade communication station in conformity with the IEEE 802.11g,the communication station transmits the whole packet according to thefirst communication method when the communication station transmits thepacket to a conventional station. On the other hand, when the high-gradecommunication station transmits a packet to a high-grade communicationstation, the transmitting communication station transmits only the firstdecoding portion of the packet according to the first communicationmethod, by which all communication stations can receive the firstdecoding portion, and can transmit the second decoding portion of thepacket including the data portion according to the second communicationmethod having a higher transmission rate.

On the transmission side of the shown packet, first, the PLCP preambleportion is transmitted as the head of the packet, and next, the SIGNALportion and the MAC frame are transmitted.

The PLOP preamble portion includes elements such as a signal detect(Signal Detect) and a channel estimation (Channel Estimation).Consequently, a peripheral station knows the existence of a signal froma communication station by receiving the PLCP preamble portion, andperforms the estimation of a transmission channel and the like.

The communication station knowing the transmission of the signal by thedetection of the PLCP preamble portion starts the reception of thesubsequently arriving SIGNAL portion. Because the SIGNAL portion istransmitted according to the first communication method, which allcommunication station know, both of the conventional stations and thehigh-grade communication stations can receive the SIGNAL portion.

The SIGNAL portion includes a transmission rate (Rate) of the subsequentMAC frame, the length (Length) of a residual data of the packet such asthe MAC frame, parity (Parity), a reserved area (Reserve) and the like.

The MAC frame is modulated according to the transmission rate specifiedby the transmission rate (Rate) of the SIGNAL portion. The MAC frame iscomposed of the MAC header and the data portion corresponding to thepayload. The MAC header describes an address (RX Address) of thereception station of the packet, Duration specifying the duration inwhich the stations other than the reception station severally shouldraise the NAV.

A communication station which can normally receive and decode the MACheader portion compares the address of the local station with thereception address. When they coincide with each other, the communicationstation receives the residual portion of the packet at a specified ratefor the duration of (packet length)/(transmission rate) in accordancewith the transmission rate (Rate) and the packet length (Length)information, both described in the SIGNAL portion. Moreover, when itsown address and the received address do not coincide with each other,the communication station raises the NAV for the Duration described inthe MAC header, and restrains any transmissions from the local station.The procedure for securing a band in accordance with the procedurementioned above is generally called as virtual carrier sense.

Now, when a transmission station of a packet being a high-gradecommunication station according to the IEEE 802.11b performs thetransmission of the packet to a high-grade communication station, thetransmission station transmits only the first decoding portion accordingto the first communication method, which all communication stations canreceive, but transmits the second decoding portion including the dataportion according to the second communication method having the highertransmission rate. Consequently, because the conventional stationscannot receive the second decoding portion, the conventional stationscannot decode the Duration described in the MAC header. Consequently,there is a problem in which the conventional stations cannot know theduration for which the conventional stations should severally raise theNAV.

Conventionally, the description of the Duration in the MAC header hasbeen used for band securing. However, for realizing the coexistence ofthe IEEE 802.11g and the IEEE 802.11a/b, a mechanism is needed for theconventional stations to recognize the duration for which theconventional stations should severally raise the NAV on the basis ofother information without using the description of the Duration.

Accordingly, the present embodiment prepares a mechanism in which, evenif a packet is transmitted according to the IEEE 802.11g as the secondcommunication method, the first decoding portion, which allcommunication stations can certainly receive, is provided, and theduration corresponding to the NAV is specified by means of the firstdecoding portion.

As shown in FIG. 3, the first decoding portion is composed of the PHYheader of a packet. Then, the period of time corresponding to theDuration is described in a pseudo-way in the SIGNAL portion, which allcommunication stations can receive, by using the information of thetransmission rate (Rate) and the packet length (Length). That is, theinformation of the transmission rate (Rate) and the packet length(Length) is spoofed so that the value of (packet length)/(rate) may beequal to the duration for which any communications are desired to bestopped.

As a result, the conventional stations severally set the packet lengthand the transmission rate, which are different from the fact, andperform the reception for a period of time corresponding to theDuration. The actual packet is not transmitted over the period of(packet length)/(rate), but the conventional stations do not start theirtransmissions for the duration corresponding to Duration. As a result,the conventional stations restrain their transmissions and continuetheir receiving for the duration for which communications are desired tobe stopped.

Incidentally, in this case, after the conventional stations haveperformed the receptions for the spoofed period of (packetlength)/(rate), CRC errors are certainly generated. The IEEE 802.11 hasa rule in which, when a CRC error is generated in the data portion, anyreceptions are restrained for a period of time EIFS longer than a normalinter frame space DIFS. Accordingly, it is desirable to perform thespoofing so that a period of time obtained by subtracting “EIFS DIFS”from the duration for which the receptions are truly desired to becontinued as the period of (packet length)/(rate) for avoiding theconventional station being always unfairly treated.

As described above, the high-grade communication stations use theinformation of the transmission rate (Rate) and the packet length(Length) so as to describe the period of time corresponding to Durationin the first decoding portion in a pseudo-way, and thereby the highgrade communication stations supply the so-called ad-hoc compatibilityto the conventional stations. In this case, for a communicationprocedure according to the high-grade communication method in conformitywith the IEEE 802.11g is being performed, the conventional stationsavoid any collisions, and thereby a normal network operation can berealized.

Moreover, in the case where the high-grade communication stations use ahigh-speed transmission rate which the first communication method cannotdeal with, a value which the first communication method can deal withshould be set in the transmission rate (Rate) field of the SIGNALportion as the spoofing in order that the conventional stations cancorrectly decode the first decoding portion. In this case, the packetlength (Length) should be also spoofed in accordance with the spoofedtransmission rate (Rate) value.

As described above, the spoofing is performed in the SIGNAL portion inorder that the value of (packet length)/(rate) may be equal to theduration for which the conventional stations are desired to stopcommunications. Hereupon the duration for which the conventionalstations are desired to stop communications, in short, indicates theduration until a communication transaction performed according to thesecond communication method ends. To put it more specific, the durationindicates the duration until an ACK transmission in a communicationprocedure performed according to the second communication method ends.Moreover, when packet transmissions are performed in a communicationprocedure for performing multiple connections with a plurality ofcommunication stations in a MAC frame according to the secondcommunication method, the above-mentioned duration corresponds to theduration until all of the ACK transmissions performed from each of theremote stations in time division multiplex end. Incidentally, JapanesePatent Application No. 2003-297919 which has been assigned to thepresent applicant, discloses a communication system in which atransmission station transmits a data frame addressed to a plurality ofreception stations by means of space division multiple access (SDMA) andeach reception station replies ACK in time division multiplex. Moreover,the transmission of the ACK packet hereupon is not limited to thetransmission of the ACK packet alone, but includes the case where theACK packet is multiplexed by the other kinds of packets such as an RTSpacket, a CTS packet and a Data packet to be transmitted.

Hereupon, it is necessary for a high-grade communication station being acommunication party to detect that the values of spoofed Rate and Lengthdescribed in the first decoding portion are spoofed for performing acorrect reception operation without performing any malfunctions based onthe spoofed Rate and Length. That is, for realizing the mechanism of thead hoc compatibility in a high-grade communication station, it is neededfor each high-grade communication station to recognize that theinformation of a packet length and a transmission rate described in thefirst decoding portion is spoofed. Moreover, for preventing theconventional stations from knowing that the information is spoofed, onlythe high-grade communication stations mutually recognize the fact, andthe first communication stations should operate in accordance with thedescription in the first decoding portion.

In the embodiment shown in FIG. 3, for example, a flag of one bitindicating the existence of the spoofing is prepared in the reservedarea (Reserve) of the SIGNAL portion. Then, when a high-gradecommunication station detect that the information of the packet lengthand the rate is spoofed by means of the flag in the first decodingportion, the high-grade communication station shifts to thecorresponding high grade rate mode, and can decode actual data byreceiving the residual packet, i.e. a high-grade decoding portion. Inthis case, the high grade communication station destroys the informationof the packet length and the rate read from the SIGNAL portion of thereceived packet.

In the case where only a single communication method (communicationmode) is defined in the second communication method for performing thepacket transmission and the reception at a high-speed transmission rate,the shift of the communication method can be specified only by means ofthe spoofed flag of one bit as described above with FIG. 3 beingreferred to. On the contrary, in the case where the second communicationmethod includes a plurality of transmission modes, it becomes impossibleto specify a transmission mode only by means of the spoofed flag of onebit.

The simplest way of specifying one of a plurality of transmission modesas described above is to add a field for specifying a transmission modein a packet. FIG. 4 shows a variation of the packet structure shown inFIG. 3. In the shown example, a SIGNAL-2 portion (high throughput (HT)PHY portion) is furthermore added after the SIGNAL portion in a packettransmitted according to the second communication method.

In the shown example, the SIGNAL-2 portion includes a field describing atrue transmission rate (True Rate) and a true packet length (TrueLength), and a field describing a mode parameter value (Mode Parameter).Because the SIGNAL-2 portion is transmitted at a fixed transmission rateat which all high-grade communication stations can perform a reception,the high-grade communication station which has received the packetperforms an reception operation in accordance with the true transmissionrate (True Rate) and the true packet length (True Length). Moreover,conventional stations cannot decode the SIGNAL-2 portion, and set theirreception duration on the basis of the rate and the length described inthe SIGNAL portion.

Now, it is needed for each of the high-grade communication stations torecognize the spoofing in the way that the conventional stations cannotknow the spoofing of the transmission rate and the packet length in theSIGNAL portion, and the conventional stations should operate inconformity with the description in the SIGNAL portion. For the sake ofthis, a packet is transmitted according to the communication method inwhich all high-grade communication stations can decode the SIGNAL-2portion (HT-SIGNAL portion) as the second communication method decodingportion and the conventional stations cannot decode the SIGNAL-2portion.

For example, the SIGNAL-2 portion is transmitted at a low transmissionrate of about 6 Mbps for all high-grade communication stations canreceive the SIGNAL-2 portion, and a modulation processing of theSIGNAL-2 portion is performed according to a modulation system whicheach of the high-grade communication stations know but the firstcommunication stations do not know. Thereby, only the high-gradecommunication stations can demodulate the SIGNAL portion to recognizethat the SIGNAL portion is spoofed.

In such a case, a high-grade communication station receiving the packettries to decode the SIGNAL-2 portion in accordance with both of thefirst communication method and a communication method which the firstcommunication stations cannot decode, and can recognize that the SIGNALportion is spoofed by the fact that the SIGNAL-2 portion can be decodedaccording to the latter method. Then, the high-grade communicationstation can perform the reception processing of the second decodingportion according to the communication mode obtained from the SIGNAL-2portion.

The SIGNAL-2 portion is located before the MAC frame being the seconddecoding portion. Consequently, in the case where the information of apacket length and a transmission rate is spoofed in the first decodingportion, a high-grade communication station receiving the packet canperform the reception operation of the second decoding portion after theSIGNAL-2 portion on the basis of the true packet length (True Length)and the true transmission rate (True Rate) describe in the SIGNAL-2portion.

A high-grade communication station performing a packet transmissionmodulates the second communication method decoding portion according toa modulation system known only by each of the high-grade communicationstations, and thereby it can be realized that all of the high-gradecommunication stations can decode the second communication methoddecoding portion, and that conventional stations cannot decode thesecond communication method decoding portion. For example, in case ofperforming a phase modulation of the SIGNAL-2 portion such as BPSK, aphase difference θ, which second communication stations jointly own, maybe given to a signal point location, or a signal point may be translatedby a known quantity Δd. On the other hand, a high-grade communicationstation receiving the packet performs the phase demodulation of thepacket in consideration of the phase shift of the signal point locationsuch as the phase difference −θ or the movement quantity −Δd. Then, thehigh-grade communication station can know the spoofing of the firstdecoding portion by the fact that the SIGNAL-2 portion could be decoded.

FIG. 16 shows an example of the inner structure of 51 the wirelessreception unit 110 in this case. The wireless reception unit 110 iscomposed of an RF unit and a PHY portion. The PHY portion is composed ofa first demodulation unit, a second demodulation unit, and a receptionprocessing unit for processing reception data which correctlydemodulated by either of these demodulation units.

The reception processing unit notifies the first demodulation unit ofthe modulation system (transmission rate) obtained from the firstdecoding portion. The first demodulation unit supposes that the firstdecoding portion is not spoofed, and demodulates the signal after thataccording to the modulation system (transmission rate) described in thefirst decoding portion by the signal point location same as that of thefirst decoding portion.

The second demodulation unit supposes that the SIGNAL-2 portion followsthe first decoding portion, and demodulates the SIGNAL-2 portionaccording to a known modulation system (transmission rate) by the signalpoint location whose phase has been rotated by 90 degrees.

The SIGNAL-2 portion has a fixed length. Consequently, when it becomesclear that the portion is the SIGNAL-2 portion after the demodulation ofa predetermined length of the SIGNAL-2 portion, it is found that thefirst decoding portion is spoofed. If not so, it is found that the firstdecoding portion is not spoofed. In the latter case, the seconddemodulation unit continues the demodulate at the unrotated signal pointlocation by the first demodulation unit. Thereby, it is possible tosuggest whether the spoofing is performed or not without providing anyspoofed flag in the reserved area (Reserve) of the first decodingportion.

Incidentally, a modulation system for providing a phase difference to asignal point on a constellation to perform mapping is, for example,disclosed in Japanese Unexamined Patent Publication No. Hei 11-146025.

The high-grade communication station can decode the second decodingportion (see DATA portion of FIG. 16) in principle, as described above.However, it is supposed that the second decoding portion cannot bedecoded when the distance between communication terminals is large, orwhen a MIMO communication is performed. In such cases, it is possible toestimate how long a packet transmission terminal directs the otherterminals to restrain their transmissions by using the first decodingportion (SIGNAL portion in FIG. 16) and the second communication methoddecoding portion (HT-SIGNAL portion in FIG. 16), both modulated at afixed low-speed rate.

The value of (packet length)/(transmission rate) calculated on the basisof the description in the SIGNAL portion as the first decoding portionis the duration until the reception of ACK in FIG. 16 is completed.Moreover, the value of (True Length)/(True Rate) calculated on the basisof the HT-SIGNAL portion as the second communication method decodingportion corresponds to the duration until the transmission of a truepacket is completed. The difference between the two (Length)/(Rate) (byadding EIF−DIFS in FIG. 16) is a value corresponding to an NAVindicating how long the packet transmission terminal directs the otherterminals to restrain their transmissions.

The method of adding the field (SIGNAL-2 portion or HT-SIGNAL portion)as shown in FIG. 4 for specifying a transmission mode to a packet forenabling the mutual notification of the transmission mode amonghigh-grade communication stations is simple, but the decrease of theoverhead and the communication efficiency caused by the transmissiondata becomes a problem.

Now, as described above, in the case where RATE and Length in the SIGNALportion are set in a pseudo-way, there are a plurality of spoofedcombinations of the packet length and the rate for indicating the sameperiod of time. For example, because the period of time necessary fortransmitting 1200 bits at 6 Mbps and the period of time necessary fortransmitting 2400 bits at 12 Mbps are the same, a reception station doesnot care which period of time is set as Rate.

However, in the case where a high-grade communication station uses ahigh-speed transmission rate which the first communication method cannotdeal with, it is necessary that a value corresponding to the firstcommunication method is spoofed in the transmission rate (Rate) field ofthe SIGNAL portion for enabling the conventional stations to decode thefirst decoding portion correctly. In this case, it is needed to performthe spoofing by adjusting the value of the packet length (Length) inorder to be able to obtain a desired Duration value according to thespoofed transmission rate (Rate) value.

In the example shown in FIG. 3, in the case where a spoofed flag is setin the SIGNAL portion being the first decoding portion, the high-gradecommunication stations destroy the information of Rate in the SIGNALportion as being spoofed. On the other hand, in the example shown inFIG. 4, it is possible to indicate which mode the successive high-grademodulation system takes by using the information of True Rate describedin the SIGNAL-2 portion.

FIG. 5 shows a description example of the Rate field in the IEEE802.11a. As shown in FIG. 5, the IEEE 802.11a sets eight transmissionrates of 6 Mbps, 9 Mbps, 12 Mbps, 18 Mbps, 24 Mbps, 36 Mbps, 48 Mbps and54 Mbps. In the Rate field, transmission rates are expressed by means offour bits. When a spoofed flag is set, it is possible to assign thedefinition of the Rate field on a standard to the specifying of anactual high-speed transfer mode.

In the example shown in FIG. 5, though the Rate field is four bits, allof the LSB's are set to be 1. Consequently, it is possible to specifyeach of 3 bits, i.e. eight modes can be specified. Moreover, the IEEE802.11b being a conventional standard includes the least upper bound ofsettable packet length (Length). Consequently, when a higher rank rateis used for spoofing, the Length field is lacked. Then, there is aproblem in which a sufficient value of Duration cannot be secured for(packet length (Length))/(rate (Rate)) (namely, an NAV of a longduration cannot be spoofed). Accordingly, actually four rates of 6 Mbps,9 Mbps, 12 Mbps, and 18 Mbps are used for the specification of thehigh-speed transfer mode for enabling the setting of large valueDuration (=(Length)/(Rate)). Because there is the possibility that thereis a conventional station which, when a Length exceeding the least upperbound is set, recognizes the information as an error and destroys theinformation, the definition is provided (the IEEE 802.11a indicates theLength information by bits, and the IEEE 802.11b indicates the Lengthinformation by periods of time).

Incidentally, because the IEEE 802.11n supposes a system using amulti-input multi-output (MIMO) communication and a system expanding acommunication use band as a high-speed transmission, a plurality oftransmission modes can exist according to the combination of the numberof antennas used for the MIMO communications and communication usebands. In such a case, the transmission mode may be notified among thehigh-grade communication stations by means of any one of theabove-described methods.

Hereupon, the MIMO communication indicates a technique for achieving theincrease of a transmission capacity and the improvement of acommunication speed by realizing space division multiplexing, i.e. aplurality of logically independent transmission paths, by providing aplurality of antenna elements both at the transmitter side and at thereceiver side. Because the MIMO communication uses the space divisionmultiplexing, frequency usability is good.

Next, a reception processing procedure of the wireless communicationapparatus 100 in the wireless network according to the presentembodiment is described.

FIG. 6 shows a reception processing procedure in the form of a flowchartin the case where the wireless communication apparatus 100 operates as aconventional station. Such a processing procedure is actually realizedin a form in which the central control unit 103 executes the instructionexecuting program stored in the information storage unit 113.

When the wireless communication apparatus 100 receives a PLOP preambleportion in step S1, the wireless communication apparatus 100successively receives the SIGNAL portion of the PHY layer in step S2.

Then, the wireless communication apparatus 100 decodes the informationof the transmission rate (Rate) and the packet length (Length) describedin the SIGNAL portion in step S3, and calculates the reception durationdetermined by (packet length)/(transmission rate).

Next, the wireless communication apparatus 100 receives a MAC headerportion at the transmission rate specified by RATE in the SIGNAL portionin step S4. Now, when the wireless communication apparatus can decodethe reception destination address on the basis of the MAC header in stepS5, the wireless communication apparatus 100 compares the receptiondestination address with the local station address in step S6. Then,when both the addresses coincide with each other, the wirelesscommunication apparatus 100 performs the reception processing for thepacket length specified by the Length of the SIGNAL portion in step S7.

Moreover, when the reception destination address and the local stationaddress do not coincide with each other in step S6, the wirelesscommunication apparatus 100 raises an NAV for the Duration determined by(packet length)/(transmission rate), and restrains its transmission instep S8.

Moreover, when the wireless communication apparatus 100 cannot decodethe reception destination address on the basis of the MAC header in stepS5, the wireless communication apparatus 100 performs receptionprocessing for a packet length specified by the Length of the SIGNALportion in step S7.

Moreover, FIG. 7 shows a reception processing procedure in the form of aflowchart when the wireless communication apparatus 100 operates as ahigh-grade communication station. Such a processing procedure isactually realized in the form in which the central control unit 103executes the instruction executing program stored in the informationstorage unit 113.

When the wireless communication apparatus 100 receives a PLCP preambleportion in step S11, the wireless communication apparatus 100successively receives the SIGNAL portion of the PHY layer in step S12.

Then, the wireless communication apparatus 100, for example, refers tothe spoofed flag in the Reserve field to judge whether the informationof the transmission rate (Rate) and the packet length (Length) isspoofed or not in step S13.

Alternatively, the wireless communication apparatus 100 judges whetherthe SIGNAL-2 portion is provided successively to the SIGNAL portion ornot. Thereby, the wireless communication apparatus judges whether theinformation of the transmission rate (Rate) and the packet length(Length) is spoofed or not in step S13. In this case, the wirelesscommunication apparatus 100 tries to demodulate the SIGNAL-2 portionaccording to the modulation system which each of the high-gradecommunication stations knows but the first communication stations do notknow in parallel with the wireless communication apparatus 100demodulates the signal after the SIGNAL-2 portion according to themodulation system (transmission rate) described in the SIGNAL portion.Then, the wireless communication apparatus 100 can recognized that theSIGNAL portion is spoofed on the basis of the fact the wirelesscommunication apparatus 100 can decode the SIGNAL-2 portion according tothe latter modulation system.

Now, when the spoofed flag is not set, the wireless communicationapparatus 100 can recognize that the packet is transmitted at thetransmission rate at which the conventional stations can receive thepacket. Then, the wireless communication apparatus 100 decodes theinformation of the transmission rate (Rate) and the packet length(Length) described in the SIGNAL portion in step S14, and calculates thereception duration determined by (packet length)/(transmission rate).

Next, the wireless communication apparatus 100 receives the MAC headerportion at the transmission rate specified by the RATE in the SIGNALportion in step S15. Now, when the wireless communication apparatus candecode the reception destination address on the basis of the MAC headerin step S16, the wireless communication apparatus 100 compares thereception destination address with the local station address in stepS17. Then, when both the addresses coincide with each other, thewireless communication apparatus 100 performs the reception processingfor the packet length specified by the Length of the SIGNAL portion instep S18.

Moreover, when the reception destination address and the local stationaddress do not coincide with each other in step S17, the wirelesscommunication apparatus 100 raises an NAV for the Duration specified bythe MAC header, and restrains its transmission in step S19.

Moreover, when the wireless communication apparatus 100 cannot decodethe reception destination address on the basis of the MAC header in stepS16, the wireless communication apparatus 100 performs receptionprocessing for a packet length specified by the Length of the SIGNALportion in step S18.

On the other hand, when the wireless communication apparatus 100 judgesthat the second decoding portion of the packet is transmitted at thetransmission rate at which only the high-grade communication stationscan receive the packet on the basis of the setting of the spoofed flagin the SIGNAL portion or on the basis of the provision of the SIGNAL-2portion in step S13, the wireless communication apparatus 100 shifts toa high speed transmission mode in step S20, and receives the MAC headerportion in step S15. The wireless communication apparatus 100 performsthe reception processing according to, for example, True Rate and TrueLength described in the SIGNAL-2 portion.

Now, when the wireless communication apparatus 100 can decode thereception destination address on the basis of the MAC header in stepS16, the wireless communication apparatus 100 compares the receptiondestination address with the local station address in step S17. Then,when both the addresses coincide with each other, the wirelesscommunication apparatus 100 performs the reception processing for thepacket length specified by the Length of the SIGNAL portion in step S18.

Moreover, when the reception destination address and the local stationaddress do not coincide with each other in step S17, the wirelesscommunication apparatus 100 raises an NAV for the Duration determined by(packet length)/(transmission rate), and restrains its transmission instep S19.

Lastly, a communication operation in the wireless network according tothe present embodiment is described. In the wireless network,conventional stations in conformity with the conventional IEEE 802.11band high-grade communication stations in conformity with the IEEE802.11g corresponding to a high-speed edition standard using the sameband as that of the IEEE 802.11b intermixedly operates.

FIG. 8 shows a communication operation example based on CSMA/CA. In theshown example, there are four communication stations #0 to #3 in acommunication environment. Among them, the communication station #0 andthe communication station #2 are supposed to be high-grade communicationstations, and the communication station #2 and the communication station#3 are supposed to be conventional stations.

Each communication station having transmission data monitors a mediumstate for a predetermined inter frame space DIFS from the last detectionof a packet. When any media are clear, namely when there are notransmission signals, the communication station performs random backoff.Furthermore, when there are no transmission signals also in this period,a transmission right is given to the communication station. In the shownexample, the communication station #0 setting the random backoff shorterthan that of the other peripheral stations acquires the transmissionright, and can start a data transmission to the communication station #1similarly as a high-grade communication station.

At the time of the data transmission, the transmission sourcecommunication station #0 transmits a first decoding portioncorresponding to the PHY header according to a first communicationmethod, which all communication stations can receive, and transmits asecond decoding portion corresponding to the MAC frame according to asecond communication method, which only the high-grade communicationstations can receive. Then, the transmission source communicationstation #0 performs the spoofing of the information of the transmissionrate (Rate) and the packet length (Length) in the SIGNAL portion of thePHY header in order that the value of (packet length)/(rate) may beequal to the duration until an ACK packet for which communications aredesired to be stopped.

Alternatively, the transmission source communication station #0transmits the SIGNAL portion in the PHY header according to the firstcommunication method, which all communication stations can receive, andsuccessively transmits the SIGNAL-2 portion modulated according to amodulation system, which each high-grade communication station knows butthe first communication stations do not know. After that, thetransmission source communication station #0 transmits the seconddecoding portion corresponding to the MAC frame according to the secondcommunication method, which only the high-grade communication stationscan receive. Then, the transmission source communication station #0performs the spoofing of the information of the transmission rate (Rate)and the packet length (Length) in the SIGNAL portion of the PHY headerin order that the value of (packet length)/(rate) may equal to theduration until the ACK packet for which communications are desired to bestopped.

The communication station #2 and the communication station #3 as theconventional stations can hear the SIGNAL portion of the packet from thecommunication station #0, and set a packet length and a transmissionrate different from the actual state to perform reception for a periodof time corresponding to the duration until the transmission of the ACKpacket ends. The data packet from the communication station #0 is nottransmitted for a period of (packet length)/(rate), but thecommunication station #2 and the communication station #3 try to receivethe data packet and do not start any transmissions. As a result, thecommunication stations #2 and #3 restrain their transmissions. Moreover,because the rate and the packet length are different from the realtransmission of the packet, the rate and the packet cannot be normallydecoded, and the communication station #2 and the communication station#3 destroy the packet.

Moreover, in the reserved area (Reserve) of the SIGNAL portion, aspoofed flag indicating the spoofing of the information of thetransmission rate (Rate) and the packet length (Length) of the SIGNALportion is set. In this case, the communication mode of a MAC frame,i.e. the true transmission rate (True Rate) and the true packet length(True Length), is indicated by a combination of Rate and Length.Alternatively, by providing the SIGNAL-2 portion, the spoofing of theinformation of the transmission rate (Rate) and the packet length(Length) of the SIGNAL portion is indicated, and the true transmissionrate (True Rate) and the true packet length (True Length) of the MACframe are described.

The communication station #1 being the communication party is ahigh-grade communication station, and detects the spoofing of theinformation of a packet length and a rate of a SIGNAL portion on thebasis of the spoofed flag. Alternatively, the communication station #1detects the spoofing of the information of the packet length and therate of a SIGNAL portion on the basis of the success of the decoding ofthe SIGNAL-2 portion. Then, the communication station #1 destroys thereception result of the SIGNAL portion in response to the detection ofthe spoofing. Furthermore, the communication station #1 receives the MACframe as the successive second decoding portion at the transmission rateindicated by the SIGNAL portion or the SIGNAL-2 portion, and performsthe reception operation of the data addressed to the local station forthe duration of Duration described in the MAC header. Then, when thedata reception is completed, the communication station #1 returns an ACKpacket to the data transmission source communication station #0.

In such a way, according to the CSMA/CA system, contention is avoidedwhile a single communication station acquires a transmission right, andany collisions can be avoided by the stop of peripheral stations' datatransmission operations during a data communication operation. Moreover,in case of inexistence of the concealed terminal problem, peripheralstations can raise NAV's to avoid collisions without passing through theRTS/CTS procedure as shown in the drawings. Thereby, overhead can bereduced.

FIG. 9 shows a communication operation example based on RTS/CTS. In theshown example, there are four communication stations #0 to #3 in acommunication environment. Among them, the communication station #0 andthe communication station #2 are supposed to be high-grade communicationstations, and the communication station #2 and the communication station#3 are supposed to be conventional stations.

Each communication station is in the following communication state. Thatis, the communication station #2 can communicate with the adjacentcommunication station #0, and the communication station #0 cancommunicate with the adjacent communication stations #1 and #2. Thecommunication station #1 can communicate with the adjacent communicationstations #0 and #3. The communication station #3 can communicate withthe adjacent communication station #1. Furthermore, the communicationstation #2 is a concealed terminal for the communication station #1, andthe communication station #3 is a concealed terminal for thecommunication station #0.

Each communication station having transmission data monitors a mediumstate for a predetermined inter frame space DIFS from the last detectionof a packet. When any media are clear, namely when there are notransmission signals, the communication station performs random backoff.Furthermore, when there are no transmission signals also in this period,a transmission right is given to the communication station. In the shownexample, the communication station #0 setting the random backoff shorterthan that of the other peripheral stations acquires the transmissionright, and can start a data transmission to the communication station #1similarly as a high-grade communication station after the inter framespace DIFS.

That is, the data transmitting communication station #0 transmits atransmission request packet (RTS) to the communication station #1. Tothis transmission, the reception destination communication station #1returns a confirmation note (CTS) to the communication station #0 afterthe shorter inter frame space SIFS (Short IFS).

Now, at the time of an RTS packet, the communication station #0transmits a first decoding portion corresponding to the PHY headeraccording to a first communication method, which all communicationstations can receive, and transmits a second decoding portioncorresponding to the MAC frame according to a second communicationmethod, which only the high-grade communication stations can receive.Then, the transmission source communication station #0 performs thespoofing of the information of the transmission rate (Rate) and thepacket length (Length) in the SIGNAL portion of the PHY header in orderthat the value of (packet length)/(rate) may be equal to the durationuntil an ACK packet for which communications are desired to be stopped.

Alternatively, the transmission source communication station #0transmits the SIGNAL portion in the PHY header according to the firstcommunication method, which all communication stations can receive, andsuccessively transmits the SIGNAL-2 portion modulated according to amodulation system, which each high-grade communication station knows butthe first communication stations do not know. After that, thecommunication station #0 transmits the second decoding portioncorresponding to the MAC frame according to the second communicationmethod, which only the high-grade communication stations can receive.Then, the communication station #0 performs the spoofing of theinformation of the transmission rate (Rate) and the packet length(Length) in the SIGNAL portion of the PHY header in order that the valueof (packet length)/(rate) may equal to the duration until the ACK packetfor which communications are desired to be stopped.

The communication station #2 as a conventional station can hear theSIGNAL portion of the RTS packet from the communication station #0, andset a packet length and a transmission rate different from the actualstate to perform a reception operation for a period of timecorresponding to (packet length)/(rate). The RTS packet from thecommunication station #0 is not transmitted for a period of (packetlength)/(rate), but the communication station #2 tries to receive thedata packet and do not start any transmissions. As a result, thecommunication station #2 restrains its transmission until thetransmission of the ACK packet is completed. Moreover, because the rateand the packet length are different from the real transmission of thepacket, the rate and the packet cannot be normally decoded, and thecommunication station #2 destroys the packet to transmitted according tothe second communication method after that.

Moreover, the reception destination communication station #1 transmitsthe first decoding portion corresponding to the PHY header according tothe first communication method, which all communication station canreceive, at the time of a transmission of a CTS packet, and transmitsthe second decoding portion corresponding to the MAC frame according tothe second communication method, which only the high-grade communicationstations can receive. Then, the communication station #1 performs thespoofing of the information of the transmission rate (Rate) and thepacket length (Length) in the SIGNAL portion of the PHY header in orderthat the value of (packet length)/(rate) may be equal to the durationuntil the ACK packet for which communications are desired to be stopped.

Alternatively, the reception destination communication station #1transmits the SIGNAL portion in the PHY header according to the firstcommunication method, which all communication stations can receive, andsuccessively transmits the SIGNAL-2 portion modulated according to amodulation system which each high-grade communication station knows butthe first communication stations do not know. After that, the receptiondestination communication station #1 transmits the second decodingportion corresponding to the MAC frame according to the secondcommunication method, which only the high grade communication stationscan receive. Then, the reception destination communication station #1performs the spoofing of the information of the transmission rate (Rate)and the packet length (Length) in the SIGNAL portion of the PHY headerin order that the value of (packet length)/(rate) may equal to theduration until the ACK packet for which communications are desired to bestopped.

The communication station #3 as the conventional station can hear theSIGNAL portion of the CTS packet from the communication station #1, andsets a packet length and a transmission rate different from the actualstate to perform reception for a period of time corresponding to theduration until the transmission of the ACK packet ends. The CTS packetfrom the communication station #1 is not transmitted for a period of(packet length)/(rate), but the communication station #3 tries toreceive the CTS packet and do not start any transmissions. As a result,the communication station #3 restrains its transmission until thecompletion of the transmission of the ACK packet. Moreover, because therate and the packet length are different from the real transmission ofthe packet, the rate and the packet length cannot be normally decoded,and the communication station #3 destroy the packet to be transmittedafter that according to the second communication method.

Then the communication station #0 starts the transmission of a datapacket in response to the reception of the CTS packet after the interframe space SIFS.

At the data transmission, the transmission source communication station#0 transmits the first decoding portion corresponding to the PHY headeraccording to the first communication method, which all communicationstations can receive, and also transmits the second decoding portioncorresponding to the MAC frame according to the second communicationmethod, which only the high-grade communication stations can receive.Then, the transmission source communication station #0 performs thespoofing of the information of the transmission rate (Rate) and thepacket length (Length) in the SIGNAL portion of the PHY header, and setsa spoofed flag indicating the spoofing.

Alternatively, the transmission source communication station #0transmits the SIGNAL portion in the PHY header according to the firstcommunication method, which all communication stations can receive, andsuccessively transmits the SIGNAL-2 portion modulated according to amodulation system, which each high-grade communication station knows butthe first communication stations do not know. After that, thetransmission source communication station #0 transmits the seconddecoding portion corresponding to the MAC frame according to the secondcommunication method, which only the high-grade communication stationscan receive. Then, the transmission source communication station #0performs the spoofing of the information of the transmission rate (Rate)and the packet length (Length) in the SIGNAL portion of the PHY headerin order that the value of (packet length)/(rate) may equal to theduration until the ACK packet for which communications are desired to bestopped.

The communication station #1 detects the spoofing of the information ofa packet length and a rate of a SIGNAL portion on the basis of thespoofed flag. Alternatively, the communication station #1 detects thespoofing of the information of the packet length and the rate of theSIGNAL portion on the basis of the success of the decoding of a SIGNAL-2portion. Then, the communication station #1 destroys the receptionresult of the SIGNAL portion in response to the detection of thespoofing. Furthermore, the communication station #1 receives the MACframe as the successive second decoding portion at the transmission rateindicated by the SIGNAL portion or the SIGNAL-2 portion, and performsthe reception operation of the data addressed to the local station forthe duration of Duration described in the MAC header. Then, when thereception of the data packet from the communication station #0 iscompleted, the communication station #1 returns an ACK packet to thedata transmission source communication station #0 after the inter framespace SIFS.

As described above, when a concealed terminal receives at least one ofthe RTS and the CTS, the concealed terminal sets a transmission stopduration of the local station for the duration in which the datatransmission based on the RTS/CTS procedure is expected to be performed,and thereby collisions can be avoided.

However, in the example shown in FIG. 9, in the case where the durationuntil the end of the RTS/CTS procedure (i.e. the duration until the ACK)is specified as the Duration, peripheral stations must wait until thelast even if the RTS/CTS procedure is broken on the way, communicationresources are wasted.

Accordingly, also a mechanism called as a Short NAV can be considered.In the Short NAV, each packet of the RTS, the CTS and data secures onlythe end of the next packet as the Duration. For example, the RTS packetis secured until the end of the CTS packet; the CTS packet is secureduntil the end of the data packet; the data packet is secured until theend of the ACK packet severally as the Duration. Consequently, even ifthe RTS/CTS procedure is broken halfway, the peripheral stations are notrequired to wait until the last.

FIG. 10 shows a communication operation example based on the RTS/CTSusing the Short NAV. Incidentally, in the shown example, a communicationenvironment similar to one shown in FIG. 9 is supposed.

Each communication station having transmission data monitors a mediumstate for a predetermined inter frame space DIFS from the last detectionof a packet. When any media are clear, namely when there are notransmission signals, the communication station performs random backoff.Furthermore, when there are no transmission signals also in this period,a transmission right is given to the communication station. In the shownexample, after the inter frame space DIFS, the communication station #0,which has the random backoff set to be shorter than that of the otherperipheral stations, acquires the transmission right to be able to starta data transmission to the communication station #1.

That is, the communication station #0, which transmits data, transmits atransmission request packet (RTS) to the communication station #1. Onthe other hand, the communication station #1 being the receptiondestination returns a confirmation note (CTS) to the communicationstation #0 after a shorter inter frame space Short IFS (SIFS).

Now, at the time of an RTS packet, the communication station #0transmits a first decoding portion corresponding to the PHY headeraccording to a first communication method, which all communicationstations can receive, and transmits a second decoding portioncorresponding to the MAC frame according to a second communicationmethod, which only the high-grade communication stations can receive.Then, the transmission source communication station #0 performs thespoofing of the information of the transmission rate (Rate) and thepacket length (Length) in the SIGNAL portion of the PHY header in orderthat the value of (packet length)/(rate) may be equal to the durationuntil an CTS packet.

Alternatively, the transmission source communication station #0transmits the SIGNAL portion in the PHY header according to the firstcommunication method, which all communication stations can receive, andsuccessively transmits the SIGNAL-2 portion modulated according to amodulation system, which each high-grade communication station knows butthe first communication stations do not know. After that, thecommunication station #0 transmits the second decoding portioncorresponding to the MAC frame according to the second communicationmethod, which only the high-grade communication stations can receive.Then, the communication station #0 performs the spoofing of theinformation of the transmission rate (Rate) and the packet length(Length) in the SIGNAL portion of the PHY header in order that the valueof (packet length)/(rate) may equal to the duration for whichcommunications are desired to be stopped.

The communication station #2 as a conventional station can hear theSIGNAL portion of the RTS packet from the communication station #0, andset a packet length and a transmission rate different from the actualstate to perform a reception operation for a period of timecorresponding to (packet length)/(rate). The RTS packet from thecommunication station #0 is not transmitted for a period of (packetlength)/(rate), but the communication station #2 tries to receive thedata packet and do not start any transmissions. As a result, thecommunication station #2 restrains its transmission until thetransmission of the CTS packet is completed. Moreover, because the rateand the packet length are different from the real transmission of thepacket, the rate and the packet cannot be normally decoded, and thecommunication station #2 destroys the packet to transmitted according tothe second communication method after that.

Moreover, the reception destination communication station #1 transmitsthe first decoding portion corresponding to the PHY header according tothe first communication method, which all communication station canreceive, at the time of a transmission of a CTS packet, and transmitsthe second decoding portion corresponding to the MAC frame according tothe second communication method, which only the high-grade communicationstations can receive. Then, the communication station #1 performs thespoofing of the information of the transmission rate (Rate) and thepacket length (Length) in the SIGNAL portion of the PHY header in orderthat the value of (packet length)/(rate) may be equal to the durationuntil the data packet.

Alternatively, the reception destination communication station #1transmits the SIGNAL portion in the PHY header according to the firstcommunication method, which all communication stations can receive, andsuccessively transmits the SIGNAL-2 portion modulated according to amodulation system which each high-grade communication station knows butthe first communication stations do not know. After that, the receptiondestination communication station #1 transmits the second decodingportion corresponding to the MAC frame according to the secondcommunication method, which only the high-grade communication stationscan receive. Then, the reception destination communication station #1performs the spoofing of the information of the transmission rate (Rate)and the packet length (Length) in the SIGNAL portion of the PHY headerin order that the value of (packet length)/(rate) may equal to theduration until the data packet for which communications are desired tobe stopped.

The communication station #3 as the conventional station can hear theSIGNAL portion of the CTS packet from the communication station #1, andsets a packet length and a transmission rate different from the actualstate to perform reception for a period of time corresponding to (packetlength)/(rate). The CTS packet from the communication station #1 is nottransmitted for a period of (packet length)/(rate), but thecommunication station #3 tries to receive the CTS packet and do notstart any transmissions. As a result, the communication station #3restrains its transmission until the completion of the transmission ofthe data packet. Moreover, because the rate and the packet length aredifferent from the real transmission of the packet, the rate and thepacket length cannot be normally decoded, and the communication station#3 destroy the packet to be transmitted after that according to thesecond communication method.

Then the communication station #0 starts the transmission of a datapacket in response to the reception of the CTS packet after the interframe space SIFS.

At the data transmission, the transmission source communication station#0 transmits the first decoding portion corresponding to the PHY headeraccording to the first communication method, which all communicationstations can receive, and also transmits the second decoding portioncorresponding to the MAC frame according to the second communicationmethod, which only the high-grade communication stations can receive.Then, the transmission source communication station #0 performs thespoofing of the information of the transmission rate (Rate) and thepacket length (Length) in the SIGNAL portion of the PHY header in orderthat the value of (packet length)/(rate) may be equal to the duration ofDuration until the ACK packet, and sets a spoofed flag indicating thespoofing.

Alternatively, the transmission source communication station #0transmits the SIGNAL portion in the PHY header according to the firstcommunication method, which all communication stations can receive, andsuccessively transmits the SIGNAL-2 portion modulated according to amodulation system, which each high-grade communication station knows butthe first communication stations do not know. After that, thetransmission source communication station #0 transmits the seconddecoding portion corresponding to the MAC frame according to the secondcommunication method, which only the high-grade communication stationscan receive. Then, the transmission source communication station #0performs the spoofing of the information of the transmission rate (Rate)and the packet length (Length) in the SIGNAL portion of the PHY headerin order that the value of (packet length)/(rate) may equal to theduration until the ACK packet for which communications are desired to bestopped.

The communication station #2 as a conventional station can hear theSIGNAL portion of the RTS packet from the communication station #0, andset a packet length and a transmission rate different from the actualstate to perform a reception operation for a period of timecorresponding to (packet length)/(rate). The data packet from thecommunication station #0 is not transmitted for a period of (packetlength)/(rate), but the communication station #2 tries to receive thedata packet and do not start any transmissions. As a result, thecommunication station #2 restrains its transmission until thetransmission of the ACK packet is completed. Moreover, because the rateand the packet length are different from the real transmission of thepacket, the rate and the packet cannot be normally decoded, and thecommunication station #2 destroys the packet to transmitted according tothe second communication method after that.

When the communication station #1 detects the spoofing of theinformation of the packet length and the rate of a SIGNAL portion on thebasis of the spoofed flag, the communication station #1 destroys theinformation. Furthermore, the communication station #1 receives the MACframe as the successive second decoding portion at the correspondingtransmission rate, and performs the reception operation of the dataaddressed to the local station for the duration of Duration described inthe MAC header. Then, when the reception of the data packet from thecommunication station #0 is completed, the communication station #1returns an ACK packet to the data transmission source communicationstation #0 after the inter frame space SIFS.

As described above, when a concealed terminal receives at least one ofthe RTS and the CTS, the concealed terminal sets a transmission stopduration of the local station for the duration in which the transmissionof the next packet is expected to be completed, and thereby collisionscan be avoided.

As described above, in the present embodiment, the high-gradecommunication stations perform the spoofing of the description of theSIGNAL portion of the PHY header, and provide the transmission stopduration to the conventional stations until a transaction according tothe high-grade communication method ends to obtain compatibility. Thatis, the conventional stations unable to deal with the high-gradecommunication method stop their transmissions for the duration in whichthe transmission of the next packet is expected to end, and therebycollisions can be avoided.

In the examples shown in FIGS. 8 and 9, in a communication procedureexecuted according to the second communication method, the spoofing ofthe description of the SIGNAL portion is performed in order that theconventional stations may stop their transmission operations for theduration until the ACK transmission ends. Moreover, when a packettransmission is performed according to a communication procedure toperform multiple connections with a plurality of communication stationsin the MAC frame according to the second communication system, the ACK(response packet) transmission is performed in a time division multiplexfrom each remote station. Also in this case, the above mentionedmechanism can be applied. Moreover, the transmission of the ACK packethereupon is not limited to the case of single ACK packet, but includes,for example, the case where the ACK packet is multiplexed with otherkinds of packets such as an RTS packet, a CTS packet and data packet tobe transmitted.

FIG. 17 shows communication operation sequencing in which a plurality ofreception stations replies by a response packet in time division to atransmission packet from a transmission station.

A packet #0 transmitted from the communication station #0 is supposed torequest a reply from the communication station #1 and the communicationstation #2 severally. The packet #0 notifies the communication station#1 and the communication station #2 of the timing of the transmissionsof their response packets lest the response packets should collide.

At this time, the value of (packet length)/(rate) of the SIGNAL portionof the packet #0 is set to be the time when the receptions of allresponse packets have been completed. Thereby, it is prevented that thecommunication station #3 locating at a position distant from thecommunication station #1 and communication station #2 to the degree ofunable to receiving the response packets from the communication stations#1 and #2 disturbs the responses. Because the SIGNAL portion istransmitted at the lowest rate, such setting is effective to eliminatesuch a concealed terminal.

Incidentally, Japanese Patent Application No. 2003-297919, which hasbeen assigned to the present applicant already, discloses acommunication system in which a transmission station transmits a dataframe addressed to a plurality of reception stations in the spacedivision multiple access (SDMA) and each reception station reply by ACKin the time division multiplex.

In the above, specific embodiments have been referred to while thepresent invention has been described in detail. However, it is clearthat the person skilled in the art can modify and substitute theembodiments without departing from the scope and spirit of the presentinvention. That is, the present invention has bee disclosed in the formof exemplifying, and the contents of the description of the presentspecification should not be interpreted limitedly. For the judgment ofthe subject matter of the present invention, claims should beconsidered.

This application is related to Japanese Priority Document No.2004-196837, filed on Jul. 2, 2004 with the Japanese Patent Office,which document is hereby incorporated by reference.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A wireless communication system comprising: a first communicationstation configured to operate according to a first communication method;a second communication station configured to operate according to boththe first communication method and a second communication method andtransmit a packet composed of a first decoding portion including spoofedinformation of a packet length and a transmission rate receivableaccording to said first communication method, and a second decodingportion receivable according to said second communication method,wherein at least a part of said second decoding portion is modulated byusing a modulation scheme that translates a signal point location ofsaid at least a part of said second decoding portion by 90 degrees phaserotation relative to a phase of said first decoding portion containingsaid spoofed information, said first decoding portion being a PHY headerincluding the spoofed information and another portion that is phaserotated relative to the phase of the first decoding portion containingthe spoofed information.
 2. A wireless communication system as cited inclaim 1, wherein: said first communication station receiving the packetcalculates (packet length)/(transmission rate) on the basis of thespoofed information of the packet length and the transmission rate ofthe packet obtained by decoding said first decoding portion in order toobtain a reception period of time for the packet.
 3. A wirelesscommunication system as cited in claim 1, wherein: when said secondcommunication station performs a communication procedure according tosaid second communication method, the second communication stationdescribes the spoofed information in a decoding portion to designate aduration by (packet length)/(transmission rate) in order to stop acommunication operation of the first communication station for thecommunication procedure within the duration.
 4. A wireless communicationsystem as cited in claim 1, wherein: said second communication stationperforming a packet transmission spoofs the information of the packetlength and the transmission rate to be described in the first decodingportion in order that the first communication station receiving thepacket stops its communication operation for the duration until acommunication transaction to be performed according to the secondcommunication method ends.
 5. A wireless communication system as citedin claim 1, wherein: said second communication station performing apacket transmission spoofs the information of the packet length and thetransmission rate to be described in the first decoding portion in orderthat the first communication station receiving the packet stops itscommunication operation for the duration until an ACK transmission endsin a communication procedure preformed according to the secondcommunication method.
 6. A wireless communication system as cited inclaim 1, wherein: when said second communication station carries out apacket transmission in accordance with a communication procedure toperform multiple connections with a plurality of communication stationsin the second decoding portion, said second communication station spoofsthe information of a packet length and a transmission rate so that thefirst communication station receiving the packet stops the communicationoperation in a duration until all of the ACK transmissions performed ina time division multiplex from each remote station end.
 7. A wirelesscommunication system as cited in claim 1, wherein: said secondcommunication method includes a plurality of communication modes; saidsecond communication station performing a packet transmission isprovided with a known second communication method decoding portion whichthe second communication station can decode, in the packet; and acommunication mode of the plurality of communication mode to be used isrepresented by the second communication method decoding portion.
 8. Awireless communication station comprising: a receiver; a transmitter,wherein said receiver and transmitter being configured to operateaccording to both a first communication method and a secondcommunication method that exchange transmissions with anothercommunication station according to a first communication method, saidanother communication station not being compatible with the secondcommunication method; and a controller configured to transmit via thetransmitter a packet comprising a first decoding portion includingspoofed information of a packet length and a transmission ratereceivable with a receiver according to the first communication method,and a second decoding portion receivable according to the secondcommunication method, wherein at least a part of said second decodingportion is modulated by using a modulation scheme that translates asignal point location of said at least a part of said second decodingportion by 90 degrees phase rotation relative to a phase of said firstdecoding portion containing said spoofed information, said firstdecoding portion being included in a PHY header and includes the spoofedinformation, the PHY header also including another portion that is partof the second decoding portion and phase rotated relative to the phaseof the first decoding portion containing the spoofed information.
 9. Awireless communication station as cited in claim 8, wherein: said firstcommunication station receiving the packet calculates (packetlength)/(transmission rate) on the basis of the spoofed information ofthe packet length and the transmission rate of the packet obtained bydecoding said first decoding portion in order to obtain a receptionperiod of time for the packet.
 10. The wireless communication station ascited in claim 8, wherein: when said second communication stationperforms a communication procedure according to said secondcommunication method, the second communication station describes thespoofed information of a packet length and a transmission rate in thefirst decoding portion to designate a duration by (packetlength)/(transmission rate) in order to stop the communication operationof the first communication station for the communication procedurewithin the duration.
 11. The wireless communication station as cited inclaim 8, wherein: said second communication station performing a packettransmission spoofs the information of the packet length and thetransmission rate to be described in the first decoding portion in orderthat the first communication station receiving the packet stops itscommunication operation for the duration until a communicationtransaction to be performed according to the second communication methodends.
 12. The wireless communication station as cited in claim 8,wherein: said second communication station performing a packettransmission spoofs the information of the packet length and thetransmission rate to be described in the first decoding portion in orderthat the first communication station receiving the packet stops itscommunication operation for the duration until an ACK transmission endsin a communication procedure preformed according to the secondcommunication method.
 13. The wireless communication system as cited inclaim 8, wherein: when said second communication station carries out apacket transmission in accordance with a communication procedure toperform multiple connections with a plurality of communication stationsin the second decoding portion, said second communication station spoofsthe information of a packet length and a transmission rate so that thefirst communication station receiving the packet stops the communicationoperation in a duration until all of the ACK transmissions performed ina time division multiplex from each remote station end.
 14. The wirelesscommunication station as cited in claim 8, wherein: said secondcommunication method includes a plurality of communication modes; saidsecond communication station performing a packet transmission isprovided with a known second communication method decoding portion whichthe second communication station can decode, in the packet; and thecommunication mode is represented by the second communication methoddecoding portion.
 15. A method for communicating in a wirelesscommunication system with a first station configured to operateaccording to a first communication method and a second communicationstation configured to operate according to both the first communicationmethod and a second communication method, comprising: transmitting fromthe second communication station a packet comprising a first decodingportion including spoofed information of a packet length and atransmission rate receivable according to the first communicationmethod, and a second decoding portion receivable according to the secondcommunication method, said transmitting step including modulating atleast a part of said second decoding portion including translating asignal point location of said at least a part of said second decodingportion by 90 degrees phase rotation relative to a phase of said firstdecoding portion containing said spoofed information, said firstdecoding portion being included in a PHY header and includes the spoofedinformation, the PHY header also including another portion that is partof the second decoding portion and phase rotated relative to the phaseof the first decoding portion containing the spoofed information.
 16. Amethod for communicating in a wireless communication system as cited inclaim 15, wherein: said first communication station receiving the packetcalculates (packet length)/(transmission rate) on the basis of thespoofed information of the packet length and the transmission rate ofthe packet obtained by decoding said first decoding portion in order toobtain a reception period of time for the packet.
 17. A method forcommunicating in a wireless communication system as cited in claim 15,wherein: when said second communication station performs a communicationprocedure according to said second communication method, the secondcommunication station describes the spoofed information of a packetlength and a transmission rate in the first decoding portion todesignate a duration by (packet length)/(transmission rate) in order tostop the communication operation of the other stations for thecommunication procedure within the duration.
 18. A method forcommunicating in a wireless communication system as cited in claim 15,wherein: said second communication station performing a packettransmission spoofs the information of the packet length and thetransmission rate to be described in the first decoding portion in orderthat the first communication station receiving the packet stops itscommunication operation for the duration until a communicationtransaction to be performed according to the second communication methodends.
 19. A method for communicating in a wireless communication systemas cited in claim 15, wherein: said second communication stationperforming a packet transmission spoofs the information of the packetlength and the transmission rate to be described in the first decodingportion in order that the first communication station receiving thepacket stops its communication operation for the duration until an ACKtransmission ends in a communication procedure preformed according tothe second communication method.
 20. A method for communicating in awireless communication system as cited in claim 15, wherein: when saidsecond communication station carries out a packet transmission inaccordance with a communication procedure to perform multipleconnections with a plurality of communication stations in the seconddecoding portion, said second communication station spoofs theinformation of a packet length and a transmission rate so that the firstcommunication station receiving the packet stops the communicationoperation in a duration until all of the ACK transmissions performed ina time division multiplex from each remote station end.
 21. A method forcommunicating in a wireless communication system as cited in claim 15,wherein: said second communication method includes a plurality ofcommunication modes; said second communication station performing apacket transmission is provided with a known second communication methoddecoding portion which the second communication station can decode, inthe packet; and the communication mode is represented by the secondcommunication method decoding portion.